1. Field of the Invention
This invention relates to forsterite (Mg.sub.2 SiO.sub.4) single crystals using therein tetravalent chromium ions as active ions for oscillating lasers and also to a method for the manufacture of said single crystals.
2. Prior Art
Solid lasers are being increasingly used in various industrial fields because of their small size, high output efficency, easy maintenance and excellent stability.
It has been reported by V. Petricevic et al. that forsterite single crystals containing chromium ions oscillate lasers at 1167-1345 nm by exciting said crystals with YAG lasers at 1064 nm (Applied Optics, Vol. 27, No. 20/15, October, 1988).
This laser oscillating wavelength of 1167-1345 nm is an extremely longer one than a laser oscillating wavelength provided by laser single crystals to which trivalent chromium ions have been added. Thus, the luminescent ions in the chromium ion-added forsterite have been considered to be tetravalent chromium ions.
It has been reported that chromium ion-added forsterite single crystals may oscillate lasers at 1200 nm and its neighborhood. In this case, however, the oscillation efficiency of said crystals is so low that they are not suitable yet for industrial use, and, therefore, the efficiency is needed to be further enhanced.
In the manufacture of tetravalent chromium ion-added forsterite single crystals, dichromium trioxide has heretofore been widely used. In the growing-up by a so-called Czochralski method, an inert atmosphere or low oxygen-content atmosphere is needed to protect expensive iridium-made crucibles for use in the growing-up. Thus, in this case, it is difficult to have trivalent chromium ions contained as tetravalent ones in the single crystals, whereas a part of chromium ions are contained rather as divalent ones therein. Thus, these divalent chromium ions cause absorption of lasers having a wavelength region including 1500-1800 nm as the central portion of the wavelength region, and this absorption remarkably decreases the efficiency of laser oscillation at about 1200 nm.
On the other hand, in a case where chromium trioxide (CrO.sub.3) is used as a chromium ion additive for forsterite, tetravalent chromium ions can be contained in forsterite; however, in this case, iridium crucibles used are oxidized or made fragile and may be cracked whereby this method is made inferior in economy and producibility.